Days to Decades: Forecasting Neutral Densities in Low Earth Orbit

The SWIMMR (Space Weather Instrumentation, Measurement, Modelling and Risk) programme is improving the UK’s capability for space weather monitoring and prediction. The thermosphere part of the programme (SWIMMRT) focuses on both short and long-term neutral density forecasting within Low Earth Orbit (LEO). For short-term forecasting up to 48 hours, the Advanced Ensemble electron density Assimilation System (AENeAS) is being made operational at the UK’s Met Office Space Weather Operations Centre (MOSWOC), with neutral densities available to end users. These improved forecasts will aid operators with conjunction analysis and re-entry predictions through better understanding of the atmospheric drag perturbation on satellite orbits. Assimilative atmospheric models fuse together global observations of the state of the atmosphere with a background model to provide a reduced error estimate of the current state. AENeAS is a 4D physics-based data assimilation model of the ionosphere-thermosphere system. It uses the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIE-GCM) as the background numerical model, solving 3D momentum, energy and continuity equations to allow physics-based nowcasting and forecasting from an observation-constrained hindcast. However, TIE-GCM has an upper boundary of between 400 and 700km, dependent upon solar activity conditions. Constituent profiles have been extrapolated above this using a Bates-Walker profile fitted to the modelled altitudes, allowing reconstruction of neutral densities throughout LEO regardless of solar activity levels. Assimilation within AENeAS is performed via a local ensemble transform Kalman filter(LETKF). Previously total election content (TEC) and ground-based ionosonde observations were assimilated into AENeAS, with the ionosphere constraining the thermosphere. We now also assimilate neutral densities derived from the energy dissipation rates (EDR) between Orbit Mean-elements Messages (OMMs). OMMs are pre-processed through use of an extended Density-Based Scattering Clustering of Applications with Noise (DBSCAN) algorithm adapted to be used with objects across the LEO regime. This identifies manoeuvres and removes outliers to provide more accurate EDR-derived densities. These neutral densities will be made available via the Met Office via gridded data, or tailored products upon request. The SWIMMR-T project also identifies that long-term forecasts currently assume a fixed climatology. However, rising carbon dioxide concentrations are leading to global cooling in the upper atmosphere, causing thermospheric contraction and reducing neutral densities at fixed altitudes. Previous studies observed a trend of between 2 and 5% reduction per decade in neutral densities at 400km, with the lower value at higher solar activity levels. The Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) has been used to quantify by how much the upper atmosphere contracts based on future ground-level carbon dioxide concentrations. The Representative Concentration Pathways (RCPs) published by the Intergovernmental Panel on Climate Change (IPCC) provide four scenarios for carbon dioxide concentration through to the year 2100. These can be used along with the WACCMX resultsto provide neutral density “forecasts” through to 2100 which account for long-term climatological change. In turn, these can be used to investigate the long-term impacts on the debris environment. The Binned Representative Atmospheric Decay (BRAD) debris model has been used to model these impacts.

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